Attempts to accurately control the relative position of a semiconductor wafer to a bake or chill plate includes capacitive proximity sensors which do not have resolution accurate enough to measure in small dimensional increments required. Reliability of capacitive proximity sensors on a hot bake or chill plate is poor.
Other attempts include placing sensors, either optical or mechanical, on the semiconductor wafer lift mechanism, thereby measuring the physical position of the mechanism and assuming that the wafer is in the proper position with reference to the bake or chill plate. Adjustment of these sensors is very difficult. Optical and mechanical sensors have a hysteresis characteristic which degrades their resolution. Also, a sensor arrangement of this kind cannot compensate for thermal expansion of parts in the mechanism, thereby making calibration very difficult.